Biology Reference
In-Depth Information
behaviour, the essentials for tight coevolution'' is certainly correct to an
extent, but many (and probably most) species do not have such tight
relationships, and coevolution for them is unlikely. Levin's (
2000
)
assumption of the existence of multiple stable states, with the possibility
of ''flips'' from one to another, also applies to closely knit systems, i.e., to
systems which are close to saturation, containing at least some species that
are common and widespread, have large SES values (Figure
11.1
), and
much potential for interaction. It is doubtful that they hold for systems
consisting of rare species with little vagility and dispersal abilities, such as
those discussed in Chapter
10
. Indeed, this is recognized by Levin (
1998
)
by distinguishing keystone species and, more importantly, ''keystone
functional groups'', that is, sets of species that control critical ecosystem
processes. Therefore, the question of whether some of the components of
ecosystems are nonsaturated and nonequilibrial may be, to some degree,
irrelevant for the ecosystem. Nevertheless, the discussion in this topic
has shown that nonsaturated, nonequilibrial systems which are not tightly
knit, are the rule rather than the exception, and tests are therefore
necessary to determine how the various models are affected by this fact.
Also, species not in any tight bondage to the system as a whole may still be
affected by the system, but the effects are largely or entirely top-down.
Thus, if there is indeed a flip from one stable state to another, it may
devastate habitats on a large scale and bring about extinction of many
''innocent'' species which have contributed nothing to the demise.
Hubbell's (
2001
) neutral theory of biodiversity and biogeography
proceeds from MacArthur and Wilson's equilibrium theory of island
biogeography, including a process of speciation and assuming neutrality
not for species but for individuals. It is limited to communities at a
particular trophic level, and makes predictions about the relative abun-
dance of species, species-area relationships, phylogeny under genetic
drift, random dispersal, and random speciation. It predicts species richness
not only on islands but on the mainland as well, and derives a fundamental
biodiversity number. It claims to reconcile the niche-assembly and
dispersal-assembly perspectives of ecological communities, the former
equilibrial and the latter nonequilibrial in nature. Furthermore, the theory
predicts that phylogenetic clades are fractal and self-similar on all taxo-
nomic scales, implying that biodiversity is fractal. Ritchie and Olff (
1999
)
have developed packing rules based on fractal geometry and have shown
that they apply to herbivorous mammals and savanna plants. Rohde
(
2001a
), however, has shown that the rules are not applicable to a large
group of parasites and probably not to the vast majority of animal species
